In these years, there is a demand for electronic devices to be fabricated with smaller size, less electric power consumption, and higher performance. In addition, there is also a demand for a non-volatile semiconductor memory that can be highly integrated, operated at high speed, and capable of maintaining data without any supply of power. A ReRAM (Resistive Random Access Memory) having a resistance variable is proposed as one of the non-volatile semiconductor memories of the next generation that can satisfy such demands. Particularly, binary transition metal oxides are proposed to be used as variable resistance films that can maintain different data values according to changes of resistance (see, for example, Non-patent documents 1 and 2).
FIG. 1A illustrates a configuration of a variable resistance device according to a related art example. The variable resistance device may also be referred to as “ReRAM device” in a case where the variable resistance device is used for a memory. The variable resistance device 100 has a variable resistance film 102 interposed between a pair of platinum (Pt) electrodes 101, 103. The variable resistance film 102 contains a transition metal(s) such as nickel oxide (NiO). FIG. 1B is a graph for describing activity of the variable resistance device 100. As indicated with arrow F in FIG. 1B, by applying a predetermined initial voltage to the variable resistance device 100, the variable resistance device 100 initiates a function of switching between two different resistance states. This initial process of applying voltage is referred to as “electroforming”. Once the electroforming is started, the variable resistance device 100 can switch between a low resistance state and a high resistance state by controlling the applied electric current or electric voltage.
More specifically, as illustrated with broken lines in FIG. 1B, by applying a voltage pulse to the variable resistance device 100 being in a high resistance state (in this example, a reset state) the IV profile of the variable resistance device 100 steeply changes at the vicinity of 1.5 V and switches to a low resistance state (in this example, a set state) (indicated with arrow a). In this example, the shift to the low resistance state is controlled to a predetermined level (indicated with arrow b) because the current is limited to a predetermined value (current limitation value). Then, the low resistance state of the variable resistance device 100 is maintained without having to apply a voltage pulse to the variable resistance device 100 (indicated with arrow c). In order to reset the variable resistance device 100 from the low resistance state to the high resistance state, one method is to cancel the limiting of the current and apply a voltage pulse of approximately 1 V. Another method is to cancel the limiting of the current and apply a current pulse of approximately 10 mA. Thereby, after exceeding the current limitation value, the resistance gradually increases (indicated with arrow d) and then abruptly switches to a high resistance state (indicated with arrow e).
Because the variable resistance film 102 of the variable resistance device 100 is an oxide material such as NiO, a precious metal resistant to oxidation (e.g., platinum (Pt), iridium (Ir)) is used to form the electrodes 101, 103 on both sides of the variable resistance film 102. Typically, however, a high voltage and a high current are required in order for an electrode formed of a precious metal to operate. Thus, it is difficult to mount an electrode formed of a precious metal on a memory device.
In the example illustrated with FIGS. 1A and 1B, the electroforming voltage (approximately 5 V) and the reset current (approximately 10 mA) are high. The electroforming voltage and the reset current of the example significantly surpass the criterion of the voltage (3.3 V or less) and the criterion of the current (1 mA or less), respectively, of the electrode that can be mounted on a memory device.
As for a variable resistance device that can achieve high speed switching, there is proposed a configuration having a TiO2/TiN nanocrystal thin film interposed between upper and lower electrodes formed of platinum (Pt) (see, for example, Non-patent document 3).
In this non-patent document 3, a TiN film having a film thickness of 200 nm is formed on a lower platinum electrode having a film thickness of 200 nm. Then, the surface of the TiN film is oxidized by annealing in an oxygen atmosphere of 400° C. for 20 minutes, to thereby form a TiO2 film on the TiN film. Then, fabrication of a ReRAM device is completed after forming an upper platinum electrode on the TiO2/TiN film.
As illustrated in FIG. 2A, in order to reduce the amount of electric current during reset, a ground side electrode (in this example, lower electrode) of a variable resistance device 10 may have a precious metal replaced with a transition metal such as nickel (Ni). A positive side electrode 13 (in this example, upper electrode) may be a platinum (Pt) electrode 13 that is resistant to oxidation. An NiO film 12 serving as a variable resistance film may be provided between the Ni electrode 11 and the Pt electrode 13. With this configuration, the current during reset can be reduced to an amount enabling the electrodes 11, 13 to be mounted on a storage device as illustrated in FIG. 2B.
However, with the configuration illustrated in FIG. 2A, corrosion due to reactive gas (e.g., chlorine (Cl2)) may occur when performing fine processes on the lower electrode 11 formed of a transition metal (e.g., Ni). This corrosion may adversely affect stability of a device (e.g., storage device).    Non-patent document 1: K. Kinoshita et al., “Bias polarity dependent data retention of resistive random access memory consisting of binary transition metal oxide”, Applied Physics Letter 89, 103509 (2006).    Non-patent document 2: S. Seo et al., “Reproducible resistance switching in polycrystalline NiO films”, Applied Physics Letter, Vol. 85, No. 23, Dec. 6, 2004.    Non-patent document 3: M. Fujimoto et al., “High-speed resistive switching of TiO2/TiN nano-crystalline thin film”, Japanese Journal of Applied Physics, Vol. 45, No. 11, 2006, pp. L310-L312.